Graphics Reference
In-Depth Information
Rasterization,” in the discussion of drawing primitives). One could also
extend the fragment shader to rotate the point sprite coordinates if
rotation of the sprite was required. This requires extra fragment shader
instructions, but increases the flexibility of the point sprite.
The texture color is attenuated by the
u_color
variable, and the alpha
value is attenuated by the particle lifetime. The application also enables
alpha blending with the following blend function:
glEnable ( GL_BLEND );
glBlendFunc ( GL_SRC_ALPHA, GL_ONE );
As a consequence of this code, the alpha produced in the fragment shader is
modulated with the fragment color. This value is then added into whatever
values are stored in the destination of the fragment. The result is an additive
blend effect for the particle system. Note that various particle effects will use
different alpha blending modes to accomplish the desired effect.
The code to actually draw the particles is shown in Example 14-8.
Example 14-8
Draw Function for Particle System Sample
void Draw ( ESContext *esContext )
{
UserData *userData = esContext−>userData;
// Set the viewport
glViewport ( 0, 0, esContext−>width, esContext−>height );
// Clear the color buffer
glClear ( GL_COLOR_BUFFER_BIT );
// Use the program object
glUseProgram ( userData−>programObject );
// Load the vertex attributes
glVertexAttribPointer ( ATTRIBUTE_LIFETIME_LOC, 1,
GL_FLOAT, GL_FALSE,
PARTICLE_SIZE * sizeof(GLfloat),
userData−>particleData );
glVertexAttribPointer ( ATTRIBUTE_ENDPOSITION_LOC, 3,
GL_FLOAT, GL_FALSE,
PARTICLE_SIZE * sizeof(GLfloat),
&userData−>particleData[1] );
glVertexAttribPointer ( ATTRIBUTE_STARTPOSITION_LOC, 3,
GL_FLOAT, GL_FALSE,
PARTICLE_SIZE * sizeof(GLfloat),
&userData−>particleData[4] );
